A series of lambda phages for DNA cloning are being constructed with the following features: (a) a wide range of DNA fragment sizes up to 20 kilo base pairs can be clones; (b) incorporation of an exogenous fragment to be indicated by the loss of the blue plaque phenotype (lac) on dye indicator plates; (c) the formation of lysogenes and, we believe, plasmids to be prevented; (d) the phage yield to be high thereby eliminating the need for growing potentially dangerous DNA clones in large volumes; (e) the exogenous segment to be transcribed under the control of a phage promoter; (f) a special method of generating deletions running into the cloned segment is to be available based on the action of gene X int.; (g) cloning can be done with either Eco Rl, Hin 3 or both restriction enzymes. These phages will greatly enhance the safety and convenience with which DNA cloning can be done. They are being used to study a number of problems including the mechanisms of DNA synthesis, transcription and packaging in phage lambda, as well as the organization of genes for ribosomal components in E. Coli. Attempts to clone preselected genes from eukaryotes are also underway. BIBLIOGRAPHIC REFERENCES: Dahlberg, James E. and Blattner, Frederick R. 1975. Sequence of the promoter-operator proximal region of the major leftward RNA of bacteriophage lambda. Nucleic Acids Research. Volume 2: 1441-1458. Barnes, Wayne M., Reznikoff, William S., Blattner, Frederick R., Dickson, Robert C. and Abelson, John. 1975. The Isolation of RNA Homologous to the Genetic Control Elements of the Lactose Operon. The Journal of Biological Chemistry. Volume 250: 8184-8191.